Understanding Plastics: Types and Manufacturing Processes
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Plastics
Plastics are materials formed by chains of carbon and hydrogen atoms. We call the chains polymers, while the atoms that form them are called monomers.
Thermoplastics
Thermoplastic materials are plastics that are softened by heating and can be formed and melted as many times as we want. This is because their polymers have weak bonds. (Polyethylene (PE), Polystyrene (PS), PVC (polyvinyl chloride))
Thermoset Plastics
The polymers of thermoset plastics criss-cross to form networks. Because of this bond, the plastics can be shaped using heat; however, once they have been formed, they cannot be re-melted to create different forms. (Melamine, Bakelite, Polyester resins)
Elastomers
Elastomers are very flexible materials that recover their shape and dimensions when external forces stop acting on them. They degrade easily with heat and cannot be re-melted once they have been formed. (Rubber, Neoprene)
Manufacturing Processes
Extrusion
Extrusion: Is used with thermoplastics to obtain long products. The plastic material is poured through a hopper onto a rotating worm. As the worm turns, it pushes the plastic through a hot tube, where it is melted. It comes out of the tube in the shape of the nozzle of the extruder. The product is then cooled.
Injection Moulding
Injection moulding: The melted plastic is injected into a cold mould, where it solidifies. Once it is cooled, the mould is opened, and the object is removed. This method is used to make toys, buckets, bowls...
Compression Moulding
Compression moulding: Is used with thermoset plastics. The plastic is placed between a mould and a counter mould. Using pressure and heat, the plastic is melted and adopts the desired shape. Is used to form small parts such as plugs.
Blow Moulding
Blow moulding: It starts with a tube that has been formed by extrusion. Air is blown inside the tube until it dilates and adopts the shape of the mould. Is used to form bottles, toys, and hollow objects.
Vacuum Forming
Vacuum forming: Is used with thermoplastic sheets that are heated and placed on a mould. The vacuum is then turned on, which forces the sheet to adapt to the shape of the mould. Is used to make yogurt pots, egg cups, layers for boxes of chocolates, etc...